I have a somewhat different problem with io_uring in practice: It's extremely hard to use /correctly/. The management of buffers which bounce across a kernel boundary and may-or-may-not end up in the same original thread lends itself to lots of subtle race conditions, resource exhaustions, and ABA issues. It's not that you can't make it work, and work well--it's that it's hard to do correctly, and very easy to make something which works 99.99% correctly, and then fails spectacularly under load or over time.

I can imagine the security implications are the same.